European Patent Specification No. 0734876A discloses a thermal printer for printing information such as dates and bar codes on packaging material, label-bearing films and other substrates by the thermal transfer of ink from a thermal print ribbon using a thermal print head having a line of selectively energizable print elements which are brought to bear against a moving ribbon with the ribbon being sandwiched between the print head and the substrate which is to bear the printed information. Conventionally, the ribbon is fed to the printing region from a supply spool by means of one or more deflection rollers, and is collected by means of one or more additional deflection rollers on a take-up spool, and once the full length of the ribbon is depleted of ink, a new spool of unused ribbon is mounted to the machine. To ease the ribbon replacement operation, spindles for receiving the supply and take-up spools are mounted on a removable cassette unit which includes the above-mentioned deflection rollers. Ribbon replacement is effected by removing the cassette unit from the printer body, removing the supply and take-up spools from the cassette unit, fitting a new supply spool with unused ribbon to the relevant spindle on the cassette unit, threading the free end of the ribbon around the deflection rollers and on to an empty take-up spool, and then sliding the cassette unit back onto the printer body, whereby the ribbon automatically takes up the correct position with respect to the thermal print head and the path of travel of the substrate material.
Ribbons of different widths may be used, depending on the transverse width of the area to be printed, that is in the direction transversely of the ribbon. However, the applicants have found that the tendency for the ribbon to break increases as the ribbon width is reduced. In addition, excessive compression of the take-up spool can occur, leading to difficulty in removing it from the printer.